The accumulation of chromosomal damage is a hallmark of cancer cells and neoplastic tissue. Whereas it is clear that certain chromosomal alterations (oncogene activation) can be well correlated with transformation of cells and the increasing malignancy of tumors, the pathway to chromosomal change is not understood. The appearance of several rare DNA repair deficiency diseases as cancer-prone syndromes suggests that disruption of normal DNA repair allows an acceleration in the rate of fixation of mutagenic events. Bloom syndrome and the 46BR syndromes are examples of DNA repair and immunodeficiency disease where patients suffer from early onset of malignancy. Both diseases are characterized by changes in DNA ligase I activity. Using a cloned mouse DNA ligase l gene, we will molecularly engineer ligase mutations into pluripotent embryonic stem cells (ES). We will monitor the impact of ligase mutation on DNA repair and recombination processes of these cells. Ligase deficient ES cells will be introduced into RAG 2- recipient mice in the formation of somatic chimeras. In lig-/RAG2- chimeras the effect of ligase activity on the expansion and selected recombination of genes of the lymphoid lineages will be evaluated in isolation. Both somatic chimeras of ligase deficient mice and germline transgenic ligase deficient mice will be a valuable model for understanding the effects of predispositipn of chromosome instability to the early events in formation of cancer cells. These animals may be expected to have elevated spontaneous mutation rates and chromosome instability, and therefore promise to be an important animal system for testing cancer prevention treatments. The germline ligase deficient mice will also be crossed with pre-existing transgenic animals containing activated translocation breakpoints (Ig/bcl2), or lacking tumor suppressor genes (p53 and Rb). In these studies we will measure the acceleration in the appearance of malignant cells and the tumor progression mechanisms as altered by a predisposing defect in ligase activity. In a second goal, we will identify genes that may interact with and regulate ligase activity in eukaryotic cells. To accomplish this objective, we will use a genetic screen in yeast, where protein-protein interactions are used to identify novel cDNAs. A detailed characterization of the role of these gene products will then be conducted, with particular attention to the regulation of DNA ligase activity and possible defects in these new genes in DNA repair deficiency diseases.